1. Field of the Invention
The present invention relates generally to the field of Magnetic Resonance (MR) magnets. More particularly, the present invention relates to pulse tube cryocooler integration and interface design for open and cylindrical MR superconducting magnets.
2. Description of the Related Art
As is well known in the art, a superconducting magnet may be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel and surrounding it with a liquid cryogen. Ultra low temperature refrigerators such as Gifford McMahon (GM) cryocoolers are widely used for maintaining the low temperature environment. The extreme cold ensures that the magnet coils are maintained in superconducting operation, such that when a power source is initially connected to the magnet coils (for a period of 10 minutes, for example) to introduce a current flow through the coils, the current will continue to flow through the coils even after the power is removed due to the absence of electrical resistance in the coils, thereby maintaining a strong magnetic field. Superconducting magnet assemblies find wide application in the field of MRI.
While GM cryocoolers are capable of providing cooling at around 4 K (liquid helium temperature), their use has several drawbacks. For one, they impart more vibrational energy to the superconducting magnets of an MRI system than is desirable, resulting in a lower image quality. Next, the acoustic signature tends to be high, resulting in complaints from doctors and technicians about coldhead chirp. In addition, GM cryocoolers have a large number of moving parts which makes them prone to frictional wear and subsequent breakdown.
In contrast to GM cryocoolers, pulse tube cryocoolers capable of providing cooling at 4 K, have far fewer drawbacks. It would be desirable to apply these cryocoolers on superconducting MR magnets, and particularly to superconducting magnets that are zero boiloff in design. Pulse tube cryocoolers offer distinct advantages for superconducting MR magnets. Pulse tubes impart much less vibrational energy to superconducting magnets than do GM cryocoolers. This improves the image quality of the MR scan and allows for more aggressive siting (i.e., allows for higher environmental/ground vibration) of the MR imaging system. The acoustic signature is less than that of a GM cryocooler, and the sound quality patterns are less annoying, resulting in a lower sound pressure level. And, pulse tube cryocoolers have far less moving parts than GM cryocoolers, which makes them more reliable.
Pulse tube cryocoolers provide unique integration challenges. Pulse tubes must be near vertically oriented (xc2x1100xc2x0) to achieve adequate cooling capacities. This creates challenges for the superconducting magnet cryostat design concerning maximum ceiling height for service and configuration of zero boiloff hardware. Zero boiloff technology requires that the cryocooler be mounted at the top of the magnet. If the recondensor is mounted directly to the pulse tube, the added height to the magnet will restrict access to the cryocooler and restrict the minimum opening through which the magnet can pass during installation. What is needed is a solution to mount the pulse tube lower while keeping the recondensor above the maximum liquid helium level. It is necessary to achieve a low thermal loss interface between the pulse tube cryocooler and a recondensor to minimize cooling power loss. It would further be desirable to eliminate the cryocooler sleeve used on conventional systems, due to the extra heat load added by the sleeve. This extra heat load requires that higher capacity cryocoolers be used, and reduces the useful life of the cryocooler.
In one aspect, the present invention describes a magnetic resonance assembly comprising, a liquid cryogen vessel, a liquid cryogen cooled superconducting magnet disposed within the liquid cryogen vessel, a closed vacuum vessel surrounding the liquid cryogen vessel and spaced from the liquid cryogen vessel, a cooling device fixably attached to the vacuum vessel operable for providing cryogenic temperatures to the superconducting magnet, a heat exchanger device in thermal contact with the liquid cryogen vessel operable for heat exchange, and a bus bar in thermal contact with the cooling device and the heat exchanger device.
In another aspect, the cooling device comprises a pulse tube cryocooler operable for generating a temperature in the range of about 4 K. The pulse tube cryocooler is connected to a remote recondensor device via a thermal bus bar of either high purity aluminum or high purity copper. The pulse tube cryocooler and remote recondensor devices are connected to the thermal bus bar using a low thermal loss interface, such as a weld, a joint, a clamp, a bolted indium joint or combinations thereof. In a further aspect, the pulse tube cryocooler may be fixably attached to the vacuum vessel as a permanent part of the magnet cryostat.
In a still further aspect, the thermal bus bar allows the pulse tube cryocooler to be attached to the vacuum vessel at any desired position on the magnet. The thermal bus bar also allows the remote recondensor device to be located at any desired position within the vacuum vessel above a maximum liquid helium level. Therefore, the thermal bus bar provides great flexibility in the design of the magnet assembly, reducing the overall height of the assembly.
In a still further aspect, the heat exchanger device is connected to the liquid cryogen vessel via one or more lines operable for transporting gas, wherein the lines allow cryogen gas to flow upward into the heat exchanger device and recondensed cryogen liquid to flow back into the liquid cryogen vessel, and provide thermal and vibration isolation between the heat exchanger device and the liquid cryogen vessel.
In a still further aspect, the present invention describes a magnetic resonance assembly comprising a liquid cryogen vessel, a liquid cryogen cooled superconducting magnet disposed within the liquid cryogen vessel, a closed vacuum vessel surrounding the liquid cryogen vessel and spaced from the liquid cryogen vessel, a means for cooling fixably attached to the vacuum vessel, a means for heat exchange in thermal contact with the liquid cryogen vessel, and a means for connecting and providing a spatial separation of the cooling means and the heat exchange means.
The present invention describes systems that allow for open and cylindrical superconducting magnets to operate using a single cryocooler without the need for coldhead switching, a cooling device that allows more aggressive siting of cylindrical magnets due to less coldhead vibration, inherently quieter operation, improved reliability, reduced magnet heat load, reduced liquid helium boiloff and lower magnet height.